A backhoe apparatus is typically mounted to a rear end of a tractor or other form of off-highway implement. Such a tractor is ordinarily equipped with a loader bucket or other tool at an opposite end of the tractor. Moreover, the tractor is steerable over the field.
A conventional backhoe apparatus includes a swing bracket or tower which attaches the backhoe apparatus to a frame of the tractor for lateral swinging movements in opposite directions and so as to allow the backhoe to assume a desired swing position relative to the frame of the tractor. The backhoe apparatus further includes a boom assembly pivotally connected at a lower end to the swing bracket and a dipper stick assembly attached toward an opposite end of the boom assembly. A bucket or other form of working tool is pivotally attached to a free end of the dipper stick assembly.
The loader bucket attached to a forward end of the tractor typically includes a pair of spaced loader arms extending from the tractor frame and having a loader bucket secured for tilting or rolling movement between the distal end of the loader arms. To further enhance the versatility of the apparatus, the bucket may be configured with a conventional two-piece clam shell design to effect a "grab" function.
A series of hydraulic motors are used to operate the loader bucket. A first pair of hydraulic motors are used to angularly position the loader arms and thereby control the elevation of the bucket relative to the tractor frame. A second pair of hydraulic motors are used to control the roll or pivotal movement of the loader bucket relative to the arms. Moreover, hydraulic motors are used for controlling articulated movements of the clam shell bucket to effect the "grab" function.
Another group or series of hydraulic motors are used to effect operation of the backhoe apparatus. The desired swing position of the backhoe apparatus is commonly controlled by a pair of hydraulic swing motors connected between the tractor frame and the swing bracket. The swing bracket and thereby the boom assembly is swung in an arcuate path about its pivotal connection to the tractor frame generally by extending one of the hydraulic motors and retracting the other hydraulic motor.
Positioning of the boom assembly relative to the swing bracket is effected by another pair of hydraulic motors. The relationship of the dipper stick assembly to the boom assembly is effected by operation of still another pair of hydraulic motors. Moreover, the position of the working tool or bucket on the backhoe apparatus is effected through actuation of still another hydraulic motor.
The ability to accurately control swinging movements of the backhoe apparatus is critical to the overall performance of the machine. When the backhoe is used in a digging operation, for example, the swing function is dramatic in that it controls the location whereat materials are discharged. Control over the swing function is also important to allow an operator to accurately return the backhoe apparatus to a precise digging position. One operation which is becoming increasingly popular with backhoes is that of using the boom assembly as a crane. As will be appreciated, accurate control of the swing function is of utmost importance where pipes or other heavy objects are to be fit into a narrow trench or the like. Particularly where the pieces being set constitute a heavy load, it is important to have complete control over the swing of the backhoe.
Heretofore known hydraulic systems used to control the swing function of a backhoe apparatus suffer serious drawbacks. Most of the known hydraulic systems provide adequate control of the swing function as long as no other hydraulic motors for the backhoe apparatus are actuated simultaneously therewith.
Effecting simultaneous backhoe functions, however, is common operator practice during operation of the backhoe apparatus. Conventional hydraulic systems will slow or even stop the swing of the backhoe apparatus when another backhoe function having a lower load requirement is performed simultaneously with the swing function. Correspondingly, the swing speed will increase when the alternative backhoe functions encounter a significant load or are stopped. The variation in swing speeds naturally occurs because the pressurized fluid in the hydraulic system takes the path of least resistance.
Operator control over swinging movements of the backhoe apparatus becomes very difficult because the swing speed can be influenced by other functions which are not related to the swing of the backhoe apparatus. Although known hydraulic systems limit maximum actuating fluid flow, they do not provide a priority flow path to the hydraulic swing motors and therefore the swing of the backhoe apparatus is subject to the flow requirements in other fluid circuits. The effects that these alternative functions have on the swing function can be compensated for by metering the various control valves used to control the various functions, but this operation requires a very skilled operator who is familiar with the machine. Alternatively, a pressure compensated flow control is used to maintain a substantially constant swing speed. As will be appreciated, the addition of a pressure-compensated flow control adds to both complexity and cost to the backhoe apparatus.
Thus, there is a need and a desire for a hydraulic system which provides a priority flow of actuating fluid to the hydraulic swing motors at a substantially constant level selected by the operator regardless of other backhoe functions simultaneously occurring therewith and their corresponding loads.